Field of the Invention
The present invention relates to a displacement detecting device that detects displacement of a surface to be measured with a noncontact sensor using light emitted from a light source, more specifically, to a technique of detecting displacement in a direction perpendicular to the surface to be measured.
Description of the Related Art
Conventionally, displacement detecting devices using light have been widely utilized as a device measuring displacement or shape of a surface to be measured in a noncontact manner. A typical example thereof includes a method of irradiating the surface to be measured with laser light to thereby detect positional changes of the reflected light by using PSD. However, this method is largely affected by a slope of the surface to be measured, and reduces sensitivity, which leads to a problem of decreasing resolution in measurement with the increase in an area to be measured.
In contrast to this, the Michelson interferometer is used by the use of the surface to be measured as a mirror. This method has a wide detection range and is excellent in linearity obtained. However, with an increase in the area to be measured, this method is affected by the change in the wavelength of a light source and the change in the index of refraction of air.
In contrast, light emitted from the light source is caused to converge on the surface to be measured with an objective lens, and the reflected light reflected on the surface to be measured is caused to converge with an astigmatic optical element and enter a light-receiving element, whereby a focus error signal is generated through an astigmatism method. Then, a servomechanism is driven using the focus error signal to thereby move the objective lens so that a focal point of the objective lens is placed on the surface to be measured. At this time, there is a system in which the displacement of the surface to be measured is detected by reading marks on a linear scale attached integrally with the objective lens via a linkage member (refer to, for example, Patent Literature 1). This method is less likely to receive an effect of variation in the slope of the surface to be measured, and is advantageous in that measurement can be performed to a large measurement area with a high resolution.
The displacement detecting device disclosed in Patent Literature 1 has an objective lens with high numerical aperture (NA) to thereby reduce the beam diameter converging on the surface to be measured, in order to detect the displacement with a high accuracy. For example, it is possible to achieve the detection accuracy on the linear scale in a range of several nanometers to several hundred nanometers by setting the beam diameter of an image formed on the surface to be measured to approximately 2 μm.